java.util.ConcurrentModificationException
       at java.util.HashMap$HashIterator.nextEntry(HashMap.java:806)
       at java.util.HashMap$EntryIterator.next(HashMap.java:843)
       at java.util.HashMap$EntryIterator.next(HashMap.java:841)
       at org.json.JSONObject.writeTo(JSONObject.java:671)
       at org.json.JSONStringer.value(JSONStringer.java:237)
       at org.json.JSONArray.writeTo(JSONArray.java:602)
       at org.json.JSONStringer.value(JSONStringer.java:233)
       at org.json.JSONObject.writeTo(JSONObject.java:672)
       at org.json.JSONObject.toString(JSONObject.java:641)
       at com.simperium.android.PersistentStore$DataStore.save(PersistentStore.java:89)
       at com.simperium.client.Bucket$1.run(Bucket.java:195)
       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
       at java.lang.Thread.run(Thread.java:841)

java.util.ConcurrentModificationException
       at java.util.HashMap$HashIterator.nextEntry(HashMap.java:806)
       at java.util.HashMap$KeyIterator.next(HashMap.java:833)
       at com.simperium.util.JSONDiff.equals(JSONDiff.java:344)
       at com.simperium.util.JSONDiff.equals(JSONDiff.java:320)
       at com.simperium.util.JSONDiff.equals(JSONDiff.java:378)
       at com.simperium.util.JSONDiff.equals(JSONDiff.java:322)
       at com.simperium.util.JSONDiff.equals(JSONDiff.java:352)
       at com.simperium.client.Syncable.isModified(Syncable.java:39)
       at com.simperium.client.Bucket$1.run(Bucket.java:194)
       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
       at java.lang.Thread.run(Thread.java:841)